I often get asked to write brand guidelines for small companies and startups as part of developing a new visual identity. Quite often I even get asked to write them before I've designed any material! This is really putting the horse before the cart – writing guidelines before material places arbitrary limitations on what you can design.
The key question to answer before writing a set of guidelines is "who is it for?".
If it's hard to think of anyone except your current designer or art director using them, then it's probably something you can worry about further down the line. Even then, it can be hard to predict needs in the future.
Most guidelines I've had to use are either prescriptive or full of unnecessary information. Here are some tips to get a useful set of guidelines together:
Describe your brand
This can be tricky, but it's a useful exercise and it's great information for anyone approaching your brand for the first time. Also, it can provide a reference point for anything you end up producing to ensure it's on-brand. You could have something as simple as 'Brand X is…' and 'Brand X isn't…'. The trick is to avoid generic and obvious terms. For example 'professional' is a word many people would like to associate with their brand, but this is already implicit as it's unlikely you'd want to appear the opposite!
Consider your outputs
Let's say you have a predominantly online business where you communicate with your audience primarily through digital channels. Then the focus of the guidelines should be mainly about how to apply the identity on screen – not much value in having grids for letterheads and flyers if you're not going to print them!
Leave some room for manoeuvre
It's very easy to get prescriptive with a set of guidelines. The upside of this is that it requires less skill to produce something on-brand. The downside is that you'll never discover anything new. When you have a new requirement it can be an opportunity to extend and improve your visual identity. I guess you can think of it a bit like a cookbook – you can follow a recipe to the letter, but it's always fun experiment with it to make something even more delicious!
Designers often worry about others 'messing up' their designs, but I think we should be more open to new thoughts and ideas on how a brand is implemented.
Don't make extra work
I've seen guidelines showing in intimate detail how to recreate a logo from scratch. What a glorious and self-indulgent waste of time! Surely the logo file in the correct format would be infinitely more useful.
If you have images and templates which should never deviate from the guidelines, it's a good idea to have them available online for your suppliers to download. That way, you can always be sure that they're working with the most up-to-date versions.